Also people can get away with re-promising things already promised; this is how Trump was played by Mexico and Canada at the beginning of his term. They glowingly offered him things the USA had already been offered that he is ignorant of.

He has a tendency to agree with the last person he spoke to who was nice to him, and he's also extremely vulnerable to flattery generally. If someone is making him happy he doesn't much care what they are saying.

The EU "deal" has convinced Trump that things have been committed by the EU that they literally do not have the power to commit.

The right wing press in the UK, even, was like, "haha, EU suckers, we got a better deal than you". And it is true, we've done very well by blowing diplomatic smoke up his arse, offering him a second state visit, generally Mandelson-ing them all.

But right wing media still tends to believe he's a great dealmaker when he is actually not; they simply didn't notice that he'd been fobbed off with undeliverables. And that means he gets the coverage from them that he is looking for.